1. Field of the Invention
This invention is in the field of methods and equipment used to reduce the emission of pollutants from diesel engines, particlarly to reduce the emission of particulate matter.
2. Background Art
In a diesel engine, liquid fuel is injected into the cylinders, which are full of compressed air at high temperature. The fuel is broken into droplets, which evaporate and mix with the air in the cylinder to form a flammable mixture. The timing of combustion is controlled by the timing of fuel injection. Major concerns include maximizing fuel efficiency, and minimizing the emission of nitrogen oxides and particulate matter.
The generation of some exhaust emissions, including particulate matter, is related to the ending time of the fuel injection interval. Generally, the earlier the fuel injection ending time, the less particulate matter is generated during the combustion cycle. So, in some diesel engines, fuel injection is timed early in the cycle to achieve full load smokeless operation, with reduced emission of particulate matter. However, early fuel injection timing increases engine cycle temperature, which results in a relatively high rate of emission of nitrogen oxides.
Conversely, one of the most effective ways to reduce NOx emissions is to retard the starting time of fuel injection. Unfortunately, the associated retardation of the fuel injection ending time causes untimely and incomplete combustion of the fuel, which, among other things, increases the emission of particulate matter. It is possible to retard the fuel injection starting time to reduce NOx emissions, while simultaneaously shortening the fuel injection duration to advance the fuel injection ending time, thereby reducing the emission of particulate matter. However, this approach increases fuel consumption. Another approach to reducing NOx emissions is to employ exhaust gas recirculation, but this also tends to increase the amount of smoke and particulate matter in the exhaust. Still another approach is to reduce NOx in the exhaust gas by after-treatment of the exhaust gas with selective catalytic reduction devices using a urea solution. However, such devices require significant changes to the structure of the vehicle to enable carrying of the urea solution, and they can result in a 3 or 4 percent reduction in fuel efficiency.
Further, when diesel engines are operated at higher altitudes, less air is introduced into the cylinders, causing the mixing of fuel and air to be less complete. This contributes to late and incomplete combustion, which increases the emission of particulate matter and smoke. Independent reduction of particulate matter in the exhaust can be achieved by particulate filters which are periodically regenerated by burning off the collected particulate matter with diesel fuel. However, this can also result in consumption of 3 or 4 percent of the fuel flow.
It would be advantageous to have a system for reducing the emission of particulate matter in diesel engine exhaust, without detracting from other means which may be employed for the reduction of NOx or other pollutants.